BTNS WG J. Touch, D. Black, Y. Wang
Internet Draft USC/ISI and EMC
Expires: December 2006 June 5, 2006
Problem and Applicability Statement
for Better Than Nothing Security (BTNS)
draft-ietf-btns-prob-and-applic-03.txt
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Abstract
The Internet network security protocol suite, IPsec, consisting of
IKE, ESP, and AH, generally requires authentication via IKE of
network layer entities to bootstrap security. This authentication can
be based on mechanisms such as pre-shared symmetric keys,
certificates and associated asymmetric keys, or the use of Kerberos.
The need to deploy authentication information and its associated
identities to network layer entities can be a significant obstacle to
use of network security. This document explains the rationale for
extending the Internet network security suite to enable use of IPsec
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security mechanisms without full IKE authentication. These extensions
are intended to protect communication in a "better than nothing"
(BTNS) fashion. The extensions may be used on their own (Stand Alone
BTNS, or SAB), or may be useful in providing network layer security
that can be authenticated by higher layers in the protocol stack,
called Channel Bound BTNS (CBB). This document also explains
situations in which use of SAB and CBB extensions are appropriate.
Table of Contents
1. Introduction...................................................3
2. Problem Statement..............................................4
2.1. Network Layer.............................................4
2.1.1. Authentication Identities............................5
2.1.2. Authentication Methods...............................5
2.1.3. Current IPsec Limits on Unauthenticated Peers........5
2.2. Upper Layer...............................................6
2.2.1. Transport Protection from Packet Spoofing............6
2.2.2. Authentication at Multiple Layers....................7
3. BTNS-IPsec Overview and Threat Models..........................9
3.1. BTNS-IPsec Overview.......................................9
3.2. BTNS-IPsec Security Services..............................9
3.3. BTNS-IPsec Modes.........................................10
4. Applicability Statement.......................................11
4.1. Symmetric SAB............................................12
4.2. Asymmetric SAB...........................................13
4.3. Symmetric CBB............................................13
4.4. Asymmetric CBB...........................................13
4.5. Benefits.................................................14
4.6. Vulnerabilities..........................................14
4.7. Summary of Uses, Vulnerabilities, and Benefits...........15
5. Security Considerations.......................................15
5.1. Threat Models and Evaluation.............................15
5.2. Interaction with Other Extant Security...................16
5.3. MITM and Masquerader Attacks.............................16
5.4. DoS Attacks and Resource Consumptions....................17
5.5. Exposure to Anonymous Access.............................18
5.6. ICMP Attacks.............................................18
5.7. Leap of Faith and Cached Credentials.....................18
5.8. Configuration Errors.....................................19
6. Other Issues and Related Efforts..............................19
6.1. NAT Traversal............................................19
6.2. Mobility and Multihoming.................................19
6.3. Related IETF Efforts.....................................19
7. IANA Considerations...........................................20
8. Acknowledgments...............................................20
9. References....................................................20
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9.1. Normative References.....................................20
9.2. Informative References...................................20
Author's Addresses...............................................22
Intellectual Property Statement..................................23
Disclaimer of Validity...........................................23
Copyright Statement..............................................23
Acknowledgment...................................................23
1. Introduction
Network security is provided by a variety of protocols at different
layers in the stack. At the network layer, the IPsec protocol suite
is used to secure IP traffic. IPsec and Internet Key Exchange
protocol (IKE) present an all-or-nothing alternative by providing
protection from a wide array of possible threats, but requiring
authentication [4][8][9][10]. In turn authentication requires
deployment of network-level authentication credentials, and this can
be an obstacle to IPsec usage. This document discusses the issues
with regard to this dependency, and introduces "Better Than Nothing
Security" (BTNS) as one solution. It also describes various modes of
BTNS, and explores the characteristics of applications that will
benefit from using each mode. The remainder of this section provides
an overview of the background and problem scenario.
The process of establishing secure network communications consists of
two functions: policy and mechanism. The policy function determines
"who" gets which types of security services, and the mechanism
function applies the selected services to the corresponding
communication traffic. The requirement for authentication credentials
stems from the need to verify the "who;" i.e., to authenticate the
identities of the communicating peers.
There are two basic approaches to authentication: using pre-deployed
information, or employing out-of-band communications. Out-of-band
authentication can be done through a trusted third party, a separate
channel to the peer, or the same channel but at a higher layer. It
requires mechanisms and interfaces to bind the authenticated
identities to the secure channels, and is out-of-scope for this
document (although it may be possible to extend the channel binding
mode of BTNS to work with such mechanisms). Pre-deployed information
includes pre-shared secrets and credentials authenticated by trusted
authorities. This form of authentication often requires manual
deployment and coordination among communicating peers. Furthermore,
authenticated credentials such as certificates signed by
certification authorities (CA) can be cumbersome and expensive to
obtain.
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These factors increase the impact of IKE's requirement for successful
authentication based on pre-deployed information before security
services are offered. Consequently, users and applications often do
not use IPsec to protect the network layer, but rely solely on higher
layer security protocols or no security at all. As the problem
statement section will describe, higher layer security protocols may
not be enough to protect against some network layer spoofing attacks.
To improve the situation, one could either reduce the hurdles to
obtain and configure authentication information, or remove
authentication at the network layer. The latter is the core idea of
BTNS, which provides anonymous (unauthenticated) keying for IPsec to
create Security Associations (SAs) with peers who do not possess
valid authentication credentials. This requires extensions to the
IPsec architecture and possibly extensions or profiles of IKE. As the
new BTNS modes in IPsec relax the authentication requirement, the
impacts, tradeoffs, and risks must be thoroughly understood before
applying BTNS to any communications. More specifically, this document
will address the issues on whether and when network layer
authentication can be removed, the risks of using BTNS, and finally,
the impacts to the existing IPsec architecture.
The next section discusses the issues that IKE's strict requirements
for network layer authentication cause for IPsec. Section 3 provides
a high level overview of BTNS-IPsec, including the security services
offered. Section 4 explores the applicability of BTNS-IPsec, followed
by a discussion of the risks and other security considerations in
Section 5. Section 6 lists other related efforts.
2. Problem Statement
This section describes the problems that motivated the development of
BTNS. The primary concern is that IPsec is not widely utilized
despite rigorous development effort and emphasis on network security
by users and organizations. There are also debates on which layer is
best for securing network communications, and how security protocols
at different layers should interact. The following discussion roughly
categorizes these issues by layers: network layer and higher layers.
2.1. Network Layer
At the network layer, one of the hurdles is to satisfy the
authentication requirements of IPsec and IKE. This section
investigates the problems on network layer authentication and the
result of this requirement.
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2.1.1. Authentication Identities
Current IPsec authentication supports several types of identities in
the Peer Authorization Database (PAD): IPv4 addresses, IPv6
addresses, DNS names, Distinguished Names, RFC 822 email addresses,
and Key IDs [10]. All require either CA-signed certificates or pre-
shared secrets to authenticate. These can be roughly categorized into
network layer identifiers and other identifiers.
The first three, IPv4/IPv6 addresses and DNS names are network layer
identifiers. The main issue with IP addresses is that they are no
longer stable identifiers representing the same physical systems
consistently due to dynamic address assignments (DHCP) and increases
in system mobility. DNS names are affected because the name to
address mapping is not always up to date as a result.
There are two main drawbacks with other, non-network-layer
identifiers. It is too restrictive because there are likely other
forms of identifiers not covered by the PAD specification. It could
also result in multiple authentications on the same identifiers at
different layers. In addition, the list of identifiers is not
complete; some higher layer protocols use additional types of
identifiers that are not supported by IPsec. These issues are also
related to channel binding and will be further discussed later.
2.1.2. Authentication Methods
As described earlier, CA-signed certificates and pre-shared secrets
are the only methods of authentications accepted by current IPsec and
IKE specifications. Pre-shared secrets require manual configuration
and out-of-band communications. The verification process of CA-signed
certificates is cumbersome and there may be a monetary cost in
obtaining the certificates. The combination of these factors is one
likely reason why IPsec is not widely used except in environments
with the highest security requirements.
2.1.3. Current IPsec Limits on Unauthenticated Peers
Pre-configuration only works if the peer identities are known in
advance. The lack of unauthenticated IPsec modes prevents secure
communications at the network layer with unauthenticated or unknown
peers, even when they are subsequently authenticated in a higher
layer protocol or application. The lack of a channel binding API
between IPsec and upper layer protocols further forces such
communications to completely bypass IPsec, leaving network layer
unprotected.
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2.2. Upper Layer
For upper layers, the following discussion first focuses on whether
IPsec is necessary if transport layer security is already in use.
This would further motivate the need to reduce the hurdle of using
IPsec. Another issue is regarding authentication at both IPsec and
higher layer protocols for the same connection.
2.2.1. Transport Protection from Packet Spoofing
Consider the case of transport protocols. Increases in network
performance and the use of long-lived connections have resulted in
increased vulnerability of connection-oriented transport protocols to
attacks. TCP, like many other protocols, is susceptible to off-path
third-party attacks, such as injection of a TCP RST [19]. The network
lacks comprehensive ingress filtering to drop such spoofed traffic.
These attacks can affect BGP sessions between core Internet routers,
and are thus of significant concern [2]. As a result, a number of
proposed solutions have been developed; most of these are transport
layer solutions.
Some of these solutions augment the transport protocol by improving
its own security, e.g., TCP/MD5 [5]. Others modify the core TCP
processing rules to make it harder for off-path attackers to inject
meaningful packets either during the initial handshake (e.g.
SYNcookies) or after a connection is established (e.g., TCPsecure)
[16][18]. Some of these modifications are new to TCP, but have
already been incorporated into other transport protocols (e.g., SCTP)
or intermediate (so-called L3.5) protocols (e.g., HIP) [12][17].
Such modifications are, at best, temporary patches to the ubiquitous
vulnerability to spoofing attacks. The obvious solution to spoofing
is to validate the segments of a connection, either at the transport
layer or the network layer. The IPsec suite already provides
authentication of a network layer packet and its contents, but the
infrastructure required for deployment of IPsec can be prohibitive.
Protecting systems from spoofed packets is ultimately an issue of
authentication, ensuring that a receiver's interpretation of the
source of a packet is accurate. Authentication validates the identity
of the source of the packet. The current IPsec suite assumes that
identity is validated either by a trusted third party - e.g., a
certification authority - or by a pre-deployed shared secret. Such an
identity is unique and invariant across associations (pair-wise
security configuration), and can be used to reject packets that are
not authentic.
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There is weaker notion of identity, one which is bootstrapped from
the session association itself. The identity doesn't mean "Bill
Smith" or "owner of shared secret X2YQ", but means something more
like "the entity with which I have been communicating on connection
#23". Such identity is not invariant across associations, but because
it is invariant within an association it can still be used to provide
protection during the lifetime of that association. This is the core
notion of identity used by BTNS.
BTNS thus provides a kind of intra-association integrity, a form of
authentication where the identity is not authenticated across
separate associations or out-of-band, but does not change during the
association. This mode of BTNS is called Stand Alone BTNS (SAB),
because the protection is afforded solely by the use of BTNS
extensions, without authentication from higher layers in the protocol
stack.
With regard to BGP in particular, it has been understood that the use
of appropriate authentication is the preferred solution [2] to TCP
spoofing attacks. Supporting authentication, e.g., by using signed
certificates, at one router does not solve the problem; that router
is still at the mercy of all routers it peers with, as it depends on
them to also support authentication. The reality is that few routers
are configured to support authentication, and the result is the use
of unsecured TCP for sending BGP packets. BTNS allows an individual
router to relax the need for authentication, in order to enable the
use of protected sessions that are not authenticated. The latter is
"better than nothing" in cases where "nothing" is the alternative.
2.2.2. Authentication at Multiple Layers
Some existing protocols used on the Internet provide authentication
at a layer above the transport, but rely on the IPsec suite for
packet-by-packet cryptographic integrity and confidentiality
services. Examples of such protocols include iSCSI and the CCM mode
for NFSv4 security [14][15]. With the current IPsec suite, the
result is two authentications; one at the IPsec layer, using an
identity for IKE and an associated secret or key, and another by the
higher layer protocol using a higher layer identity and secret or
key. This is necessary if the identity and key formats differ between
IPsec and the higher layer protocol, and because there is no standard
interface to pass authentication credentials across these layers.
End-node software is then responsible for making sure that the
identities used for these two authentications are consistent in some
fashion, an authorization policy decision. In principle a single
authentication should suffice, removing the need for:
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o the second authentication
o configuration and management of the identities and secrets or keys
for the second authentication
o determining in some fashion that the two authenticated identities
are consistent. Note that there are significant potential
vulnerabilities if this is not done.
IPsec is not always present for these higher layer protocols, and
even when present, will not always be used. Hence, if there is a
choice, the higher layer protocol authentication is preferable as it
will always be available for use independent of IPsec.
A "better than nothing" security approach to IPsec can address this
problem by setting up IPsec without an authentication and then
extending the higher layer authentication to establish that the
higher layer protocol session is protected by a single IPsec SA. This
counters man-in-the-middle (MITM) attacks on BTNS IPsec session
establishment by terminating the higher layer session when such an
attack occurs. This approach is based on the fact that an MITM attack
on a BTNS SA will result in two different BTNS SAs, each connecting
the MITM to one of the higher layer endpoints. These different SAs
contribute different cryptographic binding material to the higher
layer authentication, causing that authentication to fail, which
should then cause the higher layer protocol session to terminate. In
contrast to use of IKE authentication, this approach detects the man-
in-the-middle after the SAs have been set up, and hence does not
match IKE's resistance to denial of service attacks at the network
layer.
This check is referred in this document as "channel binding", thus
the name Channel Bound BTNS (CBB) [20]. Channel binding must be done
in a fashion that prevents a man-in-the-middle attack from changing
the SA identity when it is checked and from causing two different SAs
to have the same identity. Adding the SA identifier to
authentication mechanisms based on one-way hashes, key exchanges, or
(public key) cryptographic signatures are three means by which
channel binding can be accomplished with resistance to man-in-the-
middle attacks. This requires that the SA identifier be the same at
both ends of the SA [20].
Currently, the IPsec protocol suite does not define the notion of
channels for channel binding. Such channels can be constructed by
transport protocols layered above IP through cooperation between
these protocols and IPsec, to ensure that all packets for a given
channel are protected by similar SAs, where similar relates to, among
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other things, the IDs of the peers. Interfaces between applications
and transport protocols are also needed for communicating channel
binding data to applications, and for applications to construct their
own IPsec channels over connection-less, datagram-oriented
transports.
3. BTNS-IPsec Overview and Threat Models
This section provides an overview of BTNS-IPsec and the security
services it offers. It also describes the modes of BTNS-IPsec.
3.1. BTNS-IPsec Overview
This is an overview of what is needed to enable BTNS-IPsec. The
detailed specifications of the extensions will be addressed by the
relevant protocol specifications.
The main update to IPsec is adding extensions to security policy that
permit secure communications with un-authenticated peers. These
extensions are necessary for both IPsec and IKE. For IPsec, the
extension applies to the PAD, which specifies the forms of
authentication for each entry ID. In addition to CA-signed X.509
certificates and pre-shared secrets, the extension adds two more
categories: un-authenticated (either null or self-signed
certificates) and channel binding, to support BTNS and BTNS with
channel binding respectively. For IKE, the AUTH payload should be
expended to allow either null payload or self-signed certificates to
match the proposed PAD extensions.
The changes to enable channel binding between IPsec and higher layer
protocols or applications will be more complex than the policy
extensions above. It will involve specifications of APIs and
interactions between IPsec and higher layer protocols. This document
assumes such provisions will eventually be developed, but does not
address their details.
3.2. BTNS-IPsec Security Services
The changes and extensions of BTNS primarily affect policy as
described above. Other parts of IPsec and IKE specifications are
unchanged. BTNS-IPsec does not establish nor does it require a
separate IPsec context. It is integrated with any existing IPsec
context in a system. The scope of BTNS-IPsec applies only to the SAs
matching the policies that explicitly specify or enable BTNS modes in
the PAD. All other non-BTNS policy entries, including entries in the
SPD and the PAD, and any non-BTNS SAs will not be affected by BTNS-
IPsec in terms of security services and requirements.
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In principle, the result of removing authentication at the network
layer is that BTNS-IPsec can establish secure connections in a
fashion similar to regular IPsec and IKE, but it cannot verify or
authenticate the peer identities of these secure connections. The
following is a list of security services offered by IPsec protocol
suite. The notes address only the differences when applied to BTNS-
IPsec.
1. Access Control
Because BTNS-IPsec is integrated with any existing IPsec
contexts, the same access control mechanisms apply to BTNS-IPsec
entries in all relevant databases except that the entity IDs for
BTNS in the PAD are not authenticated. Channel bound BTNS can
authenticate after the secure connection is established at the
network layer.
2. Connectionless Integrity
3. Data Origin Authentication
4. Anti-Replay Protection
5. Confidentiality
6. (Limited) Traffic Flow Confidentiality
For the remaining security services offered by IPsec, items 2
through 6, it is possible to establish secure connections with
rogue peers in BTNS-IPsec because authentication is not required.
But once a secure connection is established, the communication is
afforded the same security services as regular IPsec.
3.3. BTNS-IPsec Modes
The previous sections have described two ways of using BTNS: Stand-
alone (SAB) or with Channel Binding (CBB). It can also be used either
symmetrically, where both parties lack network layer authentication
information, or asymmetrically, where only one party lacks the
ability to authenticate at the network layer. There are a number of
cases to consider, based on the matrix of the endpoint security
capabilities of SAB, CBB, and conventional authentication (denoted as
IKE below). The following table shows all the combinations based on
the capabilities of the two security endpoints:
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| IKE | SAB | CBB |
----+-----------+-----------+-----------+
| | | |
IKE | IKE | A-SAB | AI-CBB(*) |
| | | |
----+-----------+-----------+-----------+
| | | |
SAB | A-SAB | S-SAB | AS-CBB(*) |
| | | |
----+-----------+-----------+-----------+
| | | |
CBB | AI-CBB(*) | AS-CBB(*) | S-CBB |
| | | |
----+-----------+-----------+-----------+
1. IKE: both sides possess conventional, IKE-supported authentication
2. Symmetric SAB (S-SAB): both sides lack network layer
authentication information (and lack or do not use higher layer
authentication)
3. Asymmetric SAB (A-SAB): one side lacks network layer
authentication information (and lacks or does not use higher layer
authentication), but the other possesses it
4. Symmetric CBB (S-CBB): both sides lack network layer
authentication information, and both possess authentication at a
higher layer in the protocol stack
5. Asymmetric CBB (A-CBB): one side lacks network layer
authentication information and possesses authentication at a
higher layer in the protocol stack; there are two variants,
Asymmetric IKE CBB (AI-CBB) where the other side possesses
conventional IKE-supported authentication, and asymmetric stand-
alone CBB (AS-CBB), where the other side lacks network layer
authentication information and lacks authentication at higher
layers.
(*) Channel binding requires both ends to be bound in order to
protect against MITM attacks. Asymmetric CBB modes are listed
above for completeness. See Sec. 4.4 for further discussion.
4. Applicability Statement
BTNS protects associations once established, but does not limit with
which associations are made. It reduces vulnerability after
associations have been established to attacks from parties not
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participating in the association. This helps protect systems from
low-effort attacks on sessions or connections involving higher levels
of resources. BTNS is generally appropriate for services open to the
public but for which protected associations are desired, or for
services that can be authenticated at higher layers in the protocol
stack. BTNS can also provide some level of protection for private
services when the alternative is no protection at all (as in the case
of BGP, for instance).
BTNS-IPsec uses the IPsec protocol suite, therefore should not be
used in situations where IPsec or IKE are unsuitable. IPsec and IKE
incur additional computation overhead, and IKE further requires extra
message exchanges and round-trip times to setup security
associations. These are generally undesirable in environments with
limited computational resources and/or high communication latencies.
The following is a discussion of the applicability of each of the
combinations in the above table.
4.1. Symmetric SAB
Symmetric SAB (S-SAB) assumes that both parties lack network layer
authentication information and that authentication is not available
from higher layer protocols. S-SAB can still protect network and
transport protocols, but does not provide authentication at all. It
is useful where large files or long connections would benefit from
not being interrupted by DoS attacks, but where the particular
endpoint identities are not important.
Open services, such as web servers, and peer-to-peer networks could
utilize S-SAB when their identities need not be authenticated, but
where the communication would benefit from protection. Such services
might provide files either not validated or validated by other means
(e.g., published MD5 hashes). These transmissions present a target
for off-path attacks, which could be mitigated by the use of S-SAB.
S-SAB may also be useful for protecting the transport of voice-over-
IP (VoIP) between peers, such as direct calls between VoIP clients.
SAB is also useful in protecting any transport protocol when the
endpoints decide not to deploy authentication, for whatever reason.
This is the particular case for BGP TCP connections between core
routers, where the protection afforded by S-SAB is better than no
protection at all, even though BGP is not intended as an open
service.
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4.2. Asymmetric SAB
Asymmetric SAB (A-SAB) allows one party lacking network layer
authentication information to establish associations with another
party that possesses authentication credentials, the latter by any
applicable IKE authentication mechanisms.
Asymmetric SAB is useful for protecting transport connections for
open services on the Internet, e.g., commercial web servers, etc. In
these cases, the server is typically authenticated by a widely known
CA, as is done with TLS at the application layer, but the clients
need not be authenticated. Although this may result in IPsec and TLS
being used on the same connection, it is necessary because TLS does
not protect from certain spoofing attacks as described in the problem
statement section (e.g., TLS cannot prevent a spoofed TCP RST, as the
RST is processed by TCP instead of being passed to TLS).
A-SAB also secures transport for streaming media as would be used to
view broadcast streaming such as webcasts for remote education and
entertainment.
4.3. Symmetric CBB
Symmetric CCB (S-CCB) allows hosts without network layer
authentication information to cryptographically bind the BTNS-IPsec
channels with authentication at higher layers. This is discussed in
further detail in the channel binding specification [cite].
For higher layer protocols with full, symmetric authentication at the
application layer, e.g., TLS, pre-shared keys, etc., S-CBB allows
those protocols to use those credentials and exchanges to bind to
BTNS-IPsec, protecting the transport and network layer protocols in
addition to the application data. This enables IPsec protection at
the network layer attacks (such as TCP/RST spoofing attacks) that
would otherwise be vulnerable if only higher layer security protocols
were applied.
Note that to fully utilize S-CBB, modifications to the higher layer
authentication are necessary to incorporate the channel binding
information so that the binding cannot occur when an MITM attack
occurs.
4.4. Asymmetric CBB
As noted above, channel binding as currently proposed requires
bindings at both ends of a channel. Whether channel binding will work
asymmetrically and still offer some level of protection against MITM
- even if asymmetric - is unclear at this time. The specific
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applicability of this and other CBB modes will be discussed in detail
in a future channel bindings document.
4.5. Benefits
BTNS protects network and transport layers without requiring network
layer authentication information. It can be deployed without pre-
deployment of authentication material for IPsec or pre-shared
information, and protects all transport layer protocols using a
single mechanism.
BTNS raises the level of effort for many types of network or
transport layer attacks. Casual, simple packet attacks need to be
augmented to full associations and connection establishment for SAB,
so that an attacker must perform as much work as regular host. SAB
thus raises the effort for a DDOS attack to that of emulating a flash
crowd. For open services, there may be no way to distinguish such a
DDOS attack from a legitimate flash crowd anyway.
BTNS also allows individual associations to be protected without
requiring pre-deployed authentication credentials. We anticipate that
it will use the extant, ephemeral Diffie-Hellman exchange employed in
IKE to establish pairwise secret keys between ends of an association,
effectively removing the authentication responsibility from IKE.
4.6. Vulnerabilities
The vulnerabilities presented by BTNS depend on which variant is
supported at each end of an association. Hosts connecting to BTNS
hosts are vulnerable to communicating with a masquerader throughout
the association for SAB, or until higher layers provide additional
authentication for CBB. As a result, authentication data (e.g.,
passwords) sent to a masquerading peer could be disclosed to an
attacker. This is a deliberate design tradeoff; in BTNS, network and
transport layer access is no longer gated by the identity presented
by the other host, so this opens hosts to masquerading and flash
crowd attacks. Conversely, BTNS secures connections to hosts that
cannot authenticate at the network layer, so the network and
transport layers are more protected.
Lacking network layer authentication information, other means must be
used to provide access control for local resources. Traffic selectors
of the BTNS SPD entries can be used to limit which interfaces,
address ranges, and port ranges can access BTNS-enabled services.
Rate limiting can further restrict resource usage. For SAB, these
protections need to be considered throughout associations, whereas
for CBB they need be present only until higher layer protocols
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provide the missing authentication. CCB also relies on the
effectiveness of the binding of higher layer authentication to the
BTNS network association.
4.7. Summary of Uses, Vulnerabilities, and Benefits
The following is a summary of the properties of each type of BTNS
based on the previous subsections:
SAB CBB
-------------------------------------------------------------
Uses Open services iSCSI [14], Kerberos [11]
Peer-to-peer
Zero-config. infrastructure
Vuln. Masqueraders Masqueraders until bound
Needs data rate limit Needs conn. rate limit
Load on IPsec Load on IPsec
Exposure to open access
Benefit Protects L3 & L4 Protects L3 & L4
Avoids all auth. keys Avoids L3 auth keys
Full auth. once bound
These issues were largely noted in previous sections; some of the
more generic issues, such as the increased load on IPsec processing,
are addressed in the Security Considerations section of this
document.
5. Security Considerations
This section presents the threat models for BTNS, and discusses other
security issues based on the threat models for different modes of
BTNS. Some of the issues were mentioned previously in the document,
but are listed again for completeness.
5.1. Threat Models and Evaluation
BTNS is intended to protect sessions from a variety of threats,
including on-path, man-in-the-middle attacks after key exchange,
other on-path attacks after key exchange, and off-path attacks. It is
intended to protect the contents of a session once established using
a "leap of faith" authentication during session establishment, but
does not protect session establishment itself.
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BTNS is not intended to protect the key exchange itself, so this
presents an opportunity for a man-in-the-middle attack or a well-
timed attack from other sources. Furthermore, Stand-alone BTNS is not
intended to protect the endpoint from nodes masquerading as
legitimate clients. Channel-Bound BTNS can protect from such
masquerading, though at a later point after the security association
is established.
BTNS is also not intended to protect from DoS attacks that seek to
overload a CPU performing authentication and other security
computations, nor is it intended to protect from configuration
mistakes. These final assumptions are the same as that of the IP
network protocol security suite. Finally, manual keying is not
considered in because it is unsafe for protocols that exchange large
amounts of traffic such as IP Storage (e.g., RFC-3723 forbids use of
manual keying with the IP Storage protocols) [1].
The following sections discuss the implications of the threat models
in more details.
5.2. Interaction with Other Extant Security
As with any aspect of network security, the use of BTNS must not
interfere with extant security services. Within an IPsec context, the
scope of BTNS must be limited to the SPD and PAD entries that
explicitly specify BTNS, and to the resulting SAD entries. It is
incumbent on system administrators to deploy BTNS only where safe,
preferably as a substitute to the use of "bypass" which exempts
specified traffic from IPsec cryptograph protection. In other words,
BTNS should be used only as a substitute for no security, rather than
as a substitute for stronger security. This is particularly relevant
for the use of BTNS for BGP. Full authentication is preferred for
BGP. When that is not available, other methods, such as IP address
filtering, can help reduce the vulnerability of SAB to exposure to
anonymous access.
5.3. MITM and Masquerader Attacks
Previous sections have described how CBB can counter MITM and
masquerader attacks, even though BTNS does not protect key exchange
nor does it authenticate peer identities at the network layer.
Nonetheless, there are some security issues regarding CBB that must
be carefully evaluated before deploying BTNS.
For regular IPsec/IKE, a man in the middle cannot subvert IKE
authentication, and hence an attempt to attack it via use of two
separate security associations will cause an IKE authentication
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failure. On the other hand, a man-in-the-middle attack on IPsec with
CBB is discovered later than if IKE authentication were used. With
CBB, the BTNS-IKE step will succeed because it is unauthenticated,
and the security association will be set up. The man in the middle
will not be discovered until the higher layer authentication fails.
There are two security concerns with this approach: possible exposure
of sensitive authentication information to the attackers, and
resource consumption before attacks are detected.
The exposure of information depends on the higher layer
authentication protocols used in applications. If the higher layer
authentication requires exchange of sensitive information (e.g.,
password-derived materials) that can be attacked offline, the
attackers can gain such information even though they will be
detected. Therefore, CBB must not be used with higher layer protocols
that may expose sensitive information during authentication exchange.
For example, Kerberos V AP exchanges would leak little other than the
target's krb5 principal name, while Kerberos V AS exchanges using PA-
ENC-TIMESTAMP pre-authentication would leak material that can then be
attacked offline. The latter should not be used with BTNS, even with
Channel Binding. Further, the ways in which BTNS is integrated with
the higher layer protocol must take into consideration
vulnerabilities that could be introduced in the APIs between these
two systems or in the information that they share.
The resource consumption issue is addressed in the next section on
DoS attacks.
5.4. DoS Attacks and Resource Consumptions
BTNS deployment means that more traffic will require cryptographic
operations, which increase the load on those receiving protected
traffic and/or verifying incoming traffic. The additional computation
raises vulnerability to overloading, which can be the result of
legitimate flash crowds or from zombies utilized in DoS attacks.
Although this may itself present a substantial impediment to
deployment, it is a challenge to all cryptographically protected
communication systems, and BTNS does not create or amplify that
aspect per se. This document does not address the impact BTNS has on
such load.
The effects of the increased resource consumption are twofold. It
raises the level of effort for attackers such as MITM, but it also
consumes more resources to detect such attacks or to reject spoofed
traffic. At the network layer, proper limits or access controls for
resources should be setup for all BTNS sessions. CBB sessions can be
granted with better access once the higher layer authentications
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succeed. The same principles apply to the higher layer protocols in
the CBB sessions. Special care must be taken to avoid undue resource
usage before the authentication is established in the applications.
5.5. Exposure to Anonymous Access
The use of SAB necessarily implies that a service is being offered
for open access, since network layer authentication information is
not available. SAB must not be used with services that are not
intended to be openly available.
5.6. ICMP Attacks
This document does not consider ICMP attacks because the use of BTNS-
IPsec does not change the existing guideline [9] on how ICMP traffic
is handled. BTNS-IPsec focuses on authentication part of establishing
security associations. It does not alter the IPsec traffic processing
model and protection boundary. As a result, the entire IPsec packet
processing guidelines, including ICMP processing, remain the same for
BTNS-IPsec.
5.7. Leap of Faith and Cached Credentials
BTNS allows systems to accept and establish security associations
with peers without authenticating their identities. This resembles
the notion of "Leap of Faith" utilized in other security protocols
and applications such as SSH [21] and SSL [3].
In both SSH and SSL, implementations are allowed to accept peer
credentials (e.g., public keys or other authentication information)
without authentication, and the applications are further allowed to
store these unauthenticated credentials in local databases for future
references. Similar to BTNS, such measures are allowed due to the
lack of 'widely deployed key infrastructure' [21] and to improve ease
of use and end-user acceptance. There are still subtle differences.
Both SSH and SSL require proper warnings and options in the
applications to reject unauthenticated credentials, while BTNS will
accept those automatically if they match the corresponding policy
entries. Once a credential is accepted for the first time, it can be
cached in both SSH and SSL, and can be used automatically without
further warnings. This latter behavior matches that of BTNS, but with
different implications. SSH and SSL can use these cached,
unauthenticated credentials to verify future sessions with the same
peers. On the other hand, the key issue with BTNS-IPsec is whether
these cached credentials should be treated differently in terms of
trust levels. For SAB without Channel Binding, the cached credentials
should not be treated differently because they remain
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unauthenticated, while CBB may consider granting better access for
cached credentials with previous higher layer authentications. Note
that these should not be considered as formal recommendations because
BTNS as presently described does not consider inter-session caching
of credentials. It is left to future work to address the specifics of
cached credentials.
5.8. Configuration Errors
BTNS does not address errors of configuration that could result in
increased vulnerability; such vulnerability is already possible using
"bypass". This work presumes that associations using BTNS will
consist of SPD entries, just as "bypass," therefore separated from
associations with more conventional, stronger security.
6. Other Issues and Related Efforts
This section discusses other issues not included in any previous
categories, and lists the related work.
6.1. NAT Traversal
The issues regarding NAT traversal are mostly orthogonal to BTNS
because BTNS focuses on relaxing peer authentication in IKE and IPsec
policy. BTNS with Channel Binding may cause problems with NAT if the
IDs are tied to addresses at the application layer. Note that this
problem is not specific to BTNS, but rather to the design of generic
IPsec Channel Binding APIs. Therefore, this document does not
consider the impact of NAT or NAPT on the capabilities it intends to
provide, except as are already addressed by the current IPsec
specifications.
6.2. Mobility and Multihoming
BTNS does not consider the impact of mobility or multihoming on the
capabilities it intends to provide.
6.3. Related IETF Efforts
There are a number of related efforts in the IETF and elsewhere to
reduce the configuration effort of deploying the Internet security
suite.
PKI4IPsec is an IETF WG focused on providing an automatic
infrastructure for the configuration of Internet security services,
e.g., to assist in deploying signed certificates and CA information
[7]. The IETF KINK WG is focused on adapting Kerberos for IKE,
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enabling IKE to utilize the Kerberos key distribution infrastructure
rather than requiring certificates signed by CAs or shared private
keys [6]. KINK takes advantage of an existing architecture for
automatic key management in Kerberos. Opportunistic Encryption (OE)
is a system for automatic discovery of hosts willing to do a BTNS-
like encryption, with authentication being exchanged by leveraging
existing use of the DNS [13]. BTNS differs from all three in that
BTNS is intended to avoid the need for such infrastructure
altogether, rather than to automate it.
7. IANA Considerations
There are no IANA issues in this document.
This section should be removed by the RFC-Editor prior to final
publication.
8. Acknowledgments
This document was inspired by discussions on the IETF TCPM WG about
the recent spoofed RST attacks on BGP routers and various solutions,
as well as discussions in the nfsv4 and ips WGs about how to better
integrate with IPsec. The concept of BTNS was the result of these
discussions as well as with USC/ISI's T. Faber, A. Falk, and B. Tung,
and discussions on the IETF SAAG WG and IPsec mailing list. The
authors would like to thank the members of those WGs and lists, as
well as the IETF BTNS BOFs and WG and its associated ANONsec mailing
list (http://www.postel.org/anonsec) for their feedback, in
particular, Steve Kent, Sam Hartman, Nicolas Williams, and Pekka
Savola.
9. References
9.1. Normative References
(none)
9.2. Informative References
[1] Aboba, B., J. Tseng, J. Walker, V. Rangan, and F. Travostino,
"Securing Block Storage Protocols over IP," RFC-3723, April
2004.
[2] CERT Vulnerability Note VU#415294, "The Border Gateway Protocol
relies on persistent TCP sessions without specifying
authentication requirements," 4/20/2004.
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[3] Frier, A., P. Karlton, P. Kocher, "The SSL 3.0 Protocol",
Netscape Communications Corp., Nov 18, 1996.
[4] Harkins, D., D. Carrel, "The Internet Key Exchange (IKE),"
RFC-2409, Nov. 1998.
[5] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option," RFC-2385, Aug. 1998.
[6] IETF KINK WG web pages,
http://www.ietf.org/html.charters/kink-charter.html
[7] IETF PKI4IPSEC WG web pages,
http://www.ietf.org/html.charters/pki4ipsec-charter.html
[8] Kaufman, C., (ed.), "Internet Key Exchange (IKEv2) Protocol,"
RFC-4306, Dec. 2005.
[9] Kent, S., R. Atkinson, "Security Architecture for the Internet
Protocol," RFC-2401, Nov. 1998.
[10] Kent, S., K. Seo, "Security Architecture for the Internet
Protocol," RFC-4301, Dec. 2005.
[11] Kohl, J., C. Neuman, "The Kerberos Network Authentication
Service (V5)," RFC-1510, Sep. 1993.
[12] Mostkowitz, R., P. Nikander, P. Jokela (ed.), T. Henderson,
"Host Identity Protocol," (work in progress),
draft-ietf-hip-base-05, Mar. 2006.
[13] Richardson, M., Redelmeier, D., "Opportunistic Encryption using
The Internet Key Exchange (IKE)," RFC-4322, Dec. 2005.
[14] Satran, J., K. Meth, C. Sapuntzakis, M. Chadalapaka, E.
Zeidner, "Internet Small Computer Systems Interface (iSCSI)",
RFC-3720, April 2004.
[15] Shepler, S., B. Callaghan, D. Robinson, R. Thurlow, C., Beame,
M. Eisler, D. Noveck, "Network File System (NFS) version 4
Protocol," RFC-3530, April, 2003.
[16] Steward, R., Dalal, M., "Improving TCP's Robustness to Blind
In-Window Attacks," (work in progress),
draft-ietf-tcpm-tcpsecure-04, Feb. 2006.
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[17] Stewart, R., et al., "Stream Control Transmission Protocol,"
RFC-2960, Oct. 2000.
[18] TCP SYN-cookies, http://cr.yp.to/syncookies.html
[19] Touch, J., "Defending TCP Against Spoofing Attacks," (work in
progress), draft-ietf-tcpm-tcp-antispoof-04.txt, May. 2006.
[20] Williams, N., "On the Use of Channel Bindings to Secure
Channels," (expired, was work in progress),
draft-ietf-nfsv4-channel-bindings-03, Feb. 2005.
[21] Ylonen, T, Lonvick, C. (ed.), "The Secure Shell (SSH) Protocol
Architecture," RFC-4251, Jan. 2006.
Author's Addresses
Joe Touch
USC/ISI
4676 Admiralty Way
Marina del Rey, CA 90292-6695
U.S.A.
Phone: +1 (310) 448-9151
Email: touch@isi.edu
David Black
EMC Corporation
176 South Street
Hopkinton, MA 01748
USA
Phone: +1 (508) 293-7953
Email: black_david@emc.com
Yu-Shun Wang
USC/ISI
4676 Admiralty Way
Marina del Rey, CA 90292-6695
U.S.A.
Phone: +1 (310) 448-8742
Email: yushunwa@isi.edu
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